1. Field of the Invention
This invention relates to brazing of metal parts, and in particular, to a homogeneous, ductile brazing material useful in brazing stainless steels and superalloys.
2. Description of the Prior Art
Brazing is a process for joining metal parts, often of dissimilar composition, to each other. Typically, a filler metal that has a melting point lower than that of the base metal parts to be joined together is interposed between the metal parts to form an assembly. The assembly is then heated to a temperature sufficient to melt the filler metal. Upon cooling, a strong, corrosion resistant, leak-tight joint is formed.
Quite often the brazed assemblies are heat treated (or solution treated) after brazing. Alternatively, heat treatment or solution treatment of the base metal and brazing can be performed simultaneously. Heat treatment (or solution treatment) is a procedure that comprises heating the metal part to a preselected temperature followed by cooling at a preselected rate to achieve desired mechanical properties of the base metal. Solution treatment, frequently applied to strengthen superalloys, consists of several heating and cooling cycles. Some .gamma.' [Ni.sub.3 (Al,Ti)]hardened superalloys (e.g., lnconel 718) require solution treatment below 1010.degree. C. (1850.degree. F.) to prevent excessive grain growth and dissolution of .gamma.', resulting in reduced mechanical properties.
The most widely used brazing filler metal for joining superalloys such as lnconel 718 is a gold-nickel alloy (designated by the American Welding Society as BAu-4) consisting of 57.6 atom percent gold and 42.4 atom percent nickel (82 weight percent gold and 18 weight percent nickel). Brazing temperatures employed for this gold-nickel filler metal are in the vicinity of 996.degree. C. (1825.degree. F.), and joints formed using such filler metal provide good strength and corrosion resistance at elevated temperatures. The main drawback of this filler metal is its precious metal content, and hence its high price. For this reason, fabricators using .gamma.' superalloys have long been on the look out for less expensive substitutes. Certain of the brazing alloys designed in the AWS BNi family provide mechanical and metallurgical properties that are comparable to those of gold-nickel filler metal. However, the brazing temperatures of these BNi alloys are greater than 1010.degree. C. (1850.degree. F.). As a result, such BNi alloys are not suitable for joining .gamma.' superalloys.
Ductile glassy metal alloys have been disclosed in U.S. Pat. No. 3,856,513 issued Dec. 24, 1974 to H.S. Chen et al. These alloys include compositions having the formula M.sub.a Y.sub.b Z.sub.c, where M is a metal selected from the group consisting of iron, nickel, cobalt, vanadium and chromium, Y is an element selected from the group consisting of phosphorus, boron and carbon, and Z is an element selected from the group consisting of aluminum, silicon, tin, germanium, indium, antimony and beryllium, "a" ranges from about 60 to 90 atom percent, "b" ranges from about 10 to 30 atom percent and "c" ranges from about 0.1 to 15 atom percent. Also disclosed are glassy wires having the formula T.sub.i X.sub.j where T is a transition metal or mixture thereof and X is an element selected from the group consisting of aluminum, antimony, beryllium, boron, germanium, carbon, indium, phosphorous, silicon and tin and mixtures thereof and where the proportion in atomic percentages by i and j are respectively from about 70 to about 87 and from about 13 to about 30, with the proviso that i plus j equals 100. The transition metals T are those of Group IB, IIIB, IVB, VB, VIs, BIIV and VIII of the Periodic Chart of the Elements and include the following: scandium, yitrium, lanthanum, actinium, titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, thenium, osmium, cobalt, rhodium, iridum, nickel, palladium, platinum, copper, silver, and gold; preferably, Fe, Ni, Co, V, Cr, Pd, Pt and Ti.
There remains a need in the art for an inexpensive brazing alloy in homogeneous foil or powder form, the mechanical and metallurgical properties of which would be comparable to those of the aforesaid BAu-4 alloy.